Field of the Invention
The present invention relates to a liquid crystal display panel of a liquid crystal display device, and method of manufacturing the same.
Description of the Background Art
A TN (Twisted Nematic) mode has been widely used as a display system of a liquid crystal display panel for a liquid crystal display device. Meanwhile, a transverse electric field system in which almost a horizontal electric field is generated on a liquid crystal display panel with an application of a voltage between a pixel electrode and a counter electrode to drive liquid crystal molecules in a horizontal direction is effective for increasing a viewing angle, attaining high definition, and increasing brightness. Therefore, this system has been becoming a mainstream in especially medium or small panels represented by smartphones or tablets.
It has been known that there are an IPS (In Plane Switching) (registered trademark) mode and an FFS (Fringe Field Switching) mode in the transverse electric-field system. In the FFS mode, a lower electrode and an upper electrode, which is provided on the lower electrode via an insulating film and has a slit, are provided, wherein one of them is used as a pixel electrode and the other one is used as a counter electrode. An electric field directs from the lower electrode to a liquid crystal above the lower and upper electrodes through the slit of the upper electrode, directs in the horizontal direction (transverse direction) in the liquid crystal, and then, directs to the upper electrode located below the liquid crystal. Thus, the liquid crystal is driven with the electric field in the horizontal direction.
When a display region of a liquid crystal display panel in the FFS mode is considered in a pixel unit, a thin-film transistor is formed on a layer below an upper electrode and a lower electrode via a protection insulating film. A voltage and an external arbitrary signal (voltage) are applied to the lower electrode or the upper electrode via the thin-film transistor and a contact hole formed on the protection insulating film from a signal line. In a pixel unit, a region actually used for a display (a region used for a display in a pixel unit is referred to as a “pixel display region” hereinafter) is a region where the upper electrode and the lower electrode are superimposed with each other, and a region where the thin-film transistor, contact hole, signal line, and a scanning line are disposed is a region not used for a display (hereinafter referred to as a “non-pixel display region”). An increase in a ratio of the non-pixel display region in the pixel unit decreases an aperture ratio of the pixel display region, and this hinders implementation of a high-definition liquid crystal display panel. Therefore, it is desirable that the non-pixel display region is made as small as possible for high definition.
In addition, parasitic capacitance that becomes a cause of deterioration in display quality occurs between the lower electrode and the signal line via the protection insulating film. For this reason, an insulating film for decreasing capacitance is used for a layer below the lower electrode. For example, an organic film having small dielectric constant and capable of being formed thick is used for an insulating film. Notably, Japanese Patent Application Laid-Open No. 2010-8758 describes the liquid crystal display panel described above.
The case where a lower electrode is specified as a pixel electrode and an upper electrode is specified as a counter electrode (hereinafter referred to as a “common electrode”), for example, will be described below. A transparent conductive film is used for both the pixel electrode and the common electrode, wherein a part of the pixel electrode serving as the lower electrode is electrically connected to a thin-film transistor via a contact hole on an organic film. Regardless of such structure, in general, a transparent conductive film having relatively large resistance is formed as a thin film, while an organic film is formed thick, so that a contact hole becomes deep. Therefore, when the transparent conductive film is not uniformly formed on the inner wall of the contact hole, a desired voltage is not applied to a liquid crystal, which might cause a display defect. At present, a size of a contact hole is increased to avoid this problem. However, this results in an increase in a non-pixel display region, which hinders an increase in an aperture ratio.
In addition, the common electrode and a common line electrically connected thereto are formed on the whole display region of the liquid crystal display panel. Therefore, in a conventional structure in which they are made of a transparent conductive film having relatively large resistance, a voltage cannot uniformly be applied to the whole display region, so that a problem of an occurrence of a display defect arises.